nanoseconds is just common sub-second unit that is used. notably it is used internally in linux kernel and exposed via clock_gettime (and related functions) via timespec struct

https://man7.org/linux/man-pages/man3/timespec.3type.html

it is convenient unit because 10^9 fits neatly into 32 bit integer, and it is unlikely that anyone would need more precision than that for any general purpose use.